As the use of wireless telephone communications becomes more widespread, there is an ever increasing need to enhance the ability of transceivers to detect the wireless communication signals transmitted while minimizing the amount of bandwidth utilized.
One commonly used cellular telephony system is called code division multiple access (CDMA), wherein all cellular telephones in the system transmit their signals on a traffic channel having the same range of frequencies without regard to when other telephones are transmitting. To differentiate the transmissions to and from each cellular telephone, each telephone is associated with a unique pseudo-noise (PN) code that precedes transmissions to and from that particular telephone. To separate the signals that are designated for a particular telephone, a received signal is correlated with the telephone's unique PN code. Because each of the PN codes is generally orthogonal to all other codes in use, those signals not containing the desired PN code appear as background noise at a receiver.
In a real world cellular system, each cellular telephone receives multiple versions of a desired signal due to different paths traveled by the signals as they are transmitted between a basestation and the cellular telephone. This produces a condition known as multipath interference. To extract a desired signal from the signals that are directed to other telephones and from the multipath interference, most CDMA cellular telephones include a RAKE receiver having a number of signal paths. Each signal path correlates a differently delayed version of a received signal with the cellular telephone's unique PN code in order to extract the desired signal transmitted from a particular basestation. The outputs of each of the correlators are then further processed in a manner that attempts to undo the distortion created in the channel between a basestation and the cellular telephone.
To aid the RAKE receiver in determining the level of distortion that is introduced into the traffic channel, the CDMA basestations transmit a pilot signal having a known bit sequence on a pilot channel in addition to the data signals that are transmitted on the traffic channel. Based upon analysis of the pilot signal, the RAKE receivers can estimate the distortion of the traffic channel.
When the cellular transceiver is only receiving signals from a single basestation, the pilot signal can be used to estimate the characteristics of the traffic channel. However, in actual cellular systems, a cellular transceiver may receive signals from more than one basestation. With each basestation transmitting its own pilot signal, an estimate of the traffic channel cannot be made as readily because the characteristics of the pilot signal with respect to the traffic channel for each basestation are generally not the same. Therefore, there is a need for a technique that can accurately estimate the traffic channel characteristics when cellular signals are being received from two or more basestations in order to optimize the reception of signals in a RAKE receiver.